Re: Frustrating Problems with Gate Drive Transformers on SSTC - Ahhghhghghg! !! ! !

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "D&M's High Voltage by way of Terry Fritz <teslalist-at-qwest-dot-net>" <DMsHV.DavMcKin-at-verizon-dot-net>

Dan,

If you can, put a small gap in the core of the transformers.  This would
help prevent saturation in the transformers from the DC.  You should then be
able to remove the caps (which would block the DC).

David L. McKinnon
D&M's High Voltage



 > Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>"
<dhmccauley-at-spacecatlighting-dot-com>
 >
 > I'm having some seriously frustrating problems with the gate drive circuit
 > on my full-bridge SSTC.
 >
 > To begin with, I took a perfectly working single-ended full-bridge
 > (utilizing one-output from TL494) SSTC and converted it to push-pull mode
 > (utilizing both outputs from
 > TL494) to experiment with some audio modulation.  I decided to go bipolar
 > mode with the TL494 so I could utilize 50% of dead time to reduce output
 > power during CW
 > mode during operation.
 >
 > So in operation, half of the full-bridge would be on 25% of the time, and
 > the other half of the full bridge on for 25% with 25% deadtime between
each
 > cycle.
 >
 > Performed my modifications, checked out the TL494 driver circuit - all
good!
 >
 > Rewired my gate transformer so that now Output A of TL494 controls upper
 > left and lower right quadrant and Output B of TL494 controls lower left,
and
 > upper right quadrant of full-bridge.
 > Each control via their own gate drive transformer.
 >
 > Problem is, whenever I apply power now, one MOSFET driver (usually the
 > TC4421 of the TC4421/TC4421 pair of each output) starts to get SMOKIN'
HOT!
 > I tried disconnecting secondaries
 > on gate transformers leaving them open, replacing the TC442x chips a
million
 > times, and nothing seems to help.  If the TC442x chips are in the circuits
 > by themselves, they are fine, but as soon as you mix a pair of TC4421 and
 > TC4422 together (outputs of these go to gate transformer), then one get
 > SMOKIN HOT!
 >
 > The only thing I could think of was now that there is 75% deadtime per
 > output (as opposed to 50%) before, that maybe i have some DC bias or
 > something on the transformer primary winding.
 > I tried adding a 0.1uF capacitor in series with the primary winding of the
 > gate transformer and it fixed this problem.  Now the TC442x run cool and
 > seem to work.
 >
 > HOWEVER!  Always a however . . .   Now my gate drive signals are all
messed
 > up - basically probably because of the series capacitor now.
 > They look a bit like this:
 >
 > http://users.snip-dot-net/~liche/waveform.jpg
 >
 > If anyone has any comments on what I may be doing wrong, or what I can
 > possibly do to fix this problem, I'd appreciate it.
 > I was expecting to do this modification in one hour and have it ready to
run
 > and try audio modulating it this evening, but now 4 hours later, I'm about
 > ready to
 > throw the damn thing out the window.  Oh well . . . it always works that
 > way!
 >
 > My schematic is here, although the gate transformers are still wired for
 > previous working configuration, not the new push-pull arrangement.
 > http://www.spacecatlighting-dot-com/datasheets/sstc2-pwmdriver.pdf
 > http://www.spacecatlighting-dot-com/datasheets/sstc2-hbridge.pdf
 >
 > Thanks
 > The Captain
 >
 >